The continuously variable friction belt transmission offers several desirable advantages over the gear type of power transmission. A primary advantage is its apparent capability to provide stepless changes in torque and speed output ratios. Such belt transmissions are conventionally comprised of two pulleys mounted on power input and output shafts. On opposite sides of the belt, each pulley has an axially movable pulley half for varying the effective drive diameter and effecting changes in the speed and torque ratios. Such a simple concept would appear to be most desirable. Yet, because of control problems, this apparent capability is somewhat elusive. Existing control systems for belt transmissions are either complicated or lack the refinement required to achieve ratio changes without continuous maintenance and readjustment to eliminate development of belt slippage and consequent power losses. For these reasons, change speed gear drives are still preferred even in lower power applications. Thus, even multi-speed bicycles continue to utilize a plurality of gears and a sophisticated derailler system.
A friction belt transmission which would overcome the prior art difficulties and receive greater acceptance would incorporate the following advantages:
1. The torque and speed ratios would be capable of positive, finite control.
2. Differential rotation of the pulley halves, inertial forces and vibration would not effect undesired changes in the torque and speed ratios.
3. Belt slippage would be essentially precluded.
4. Speed and torque ratio changes could be made with minimum force requirements.
5. The unit would be compact, minimizing the dimensions transverse to the pulleys for incorporation into environments such as bicycles.
6. The transmission would be of low cost and simple in design, minimizing maintenance, and eliminating adjustments otherwise required to maintain belt tension.
7. Friction and drag effects of the belt upon the pulley halves would be minimized.
8. The design criteria could readily be changed to achieve any desired rate of change of speed ratios.
9. The transmission would be especially suited for incorporation into a bicycle.
10. The rate of change of increasing the torque ratio can be different from the rate of change of decreasing the torque ratio.